/Engine/Source/Runtime/Core/Public/HAL/MallocBinned.h
For some allocators this will return the actual size that should be requested to eliminate internal fragmentation. The return value will always be >= Count. This can be used to grow and shrink containers to optimal sizes. This call is always fast and threadsafe with no locking.
virtual SIZE_T QuantizeSize ( SIZE_T Count, uint32 Alignment )